This invention relates generally to a printed circuit board having self-contained printed resistors, and more particularly to a printed circuit board with self-contained printed resistors which is capable of reducing the influence by noises.
Because of a small-size design and a multi-function design of electronic devices, printed circuit boards have now been directed toward a higher density design. Such a high-density packaging is made, for example, by the narrowing of the pattern of a conductor circuit, a multi-layer construction, the reduction of a diameter of through holes including an interstitial via hole, such as a blind hole and a barried hole, and the mounting of small-size chips on the surface of the board.
As one of such high-density packaging (mounting) techniques, there has been developed a printed resistor. For forming such a printed resistor, a paste for a polymer thick-film resistor is applied, for example, by screen printing to a surface of an insulating board so as to extend between terminals of conductor circuits formed in a predetermined pattern on the insulating board, and then this paste is thermoset to provide the printed resistor. This printed resistor is useful for a high-density arrangement of electronic parts where it is difficult to mount separate resistors on the insulating board, and therefore the printed resistor is advantageous for a high-density packaging. And besides, since the printed resistor is very thin as a resistor, it can advantageously be formed beneath other parts, and can be arranged to intersect jumper wires for the conductor paste in a three-dimensional manner via an insulating coat layer. Further, the printed resistor can be formed by screen printing enabling mass-production, and therefore the printed resistors have been widely used in a single-sided circuit board, a double-sided circuit board and a hybrid circuit board for livelihood devices, such as a radio and a recorder.
Because of the above high-density arrangement of the pattern of the printed circuit board and the above high-density mounting of the parts and also because of an increased application of information equipment for livelihood use, multi-layer printed circuit boards have been increasingly used also in livelihood devices. From now on, it is thought that the high-density design, achieved by the narrowing of the pattern of the circuit conductor, the reduction of the diameter of the through holes, and the mounting of small-size chips on the board surface (particularly, a direct mounting technique), will make further progress in this field.
With respect to the technique of mounting the chips on the surface among these techniques, fine chips are connected directly to the circuit pattern on the printed circuit board without the use of leads, so that the number of parts to be mounted on a given area can be increased, and also the parts can be mounted on both of the front and reverse surfaces of the printed board, thereby enhancing the mounting density.
However, it is not always advisable that the above technique useful for the high-density design be applied to a multi-layer printed circuit board for reasons given below. Namely, the area of the circuit board can be reduced by the multi-layer structure, but the chips can be mounted only on the two outermost layers (i.e., the opposite sides) of the circuit board, and therefore in many cases the minimum area required for the circuit board is limited by the number of the chips to be mounted. With respect to this problem, although the number of the chips to be mounted can be reduced by forming separate circuits into an integrated circuit (IC), this IC formation requires the accumulation of technology, and also can be practiced only for the purpose of mass-production. Thus, the IC formation is not easy, and poses a problem that the cost is increased.
Generally, the number of resistors used in a livelihood circuit board is about 1/3 to 1/2 of all the parts. Therefore, if the resistors among the chips to be mounted are formed into the above-mentioned thin-film printed resistors, and are combined with other printed wiring board so as to be contained in a multi-layer printed circuit board, the above problem can be considerably solved. This is true not only with the resistors among the chips to be mounted on the board, but also with discrete resistors which are connected to the conductor circuit by their lead wires passed through parts holes in the printed board.
One example of the prior art proposed based on this idea is a multi-layer printed circuit board in which an inner-layer board, having printed resistors formed by a paste for a polymer thick-film resistor, is combined with prepregs and wiring boards to provide a laminate construction which is integrated under heat and pressure to provide the multi-layer printed circuit board (Japanese Patent Unexamined Publication No. 60-263499). It has also been proposed that when forming this multi-layer structure, the surface of the printed resistor mounted on the inner-layer board is coated with a protective coat composed of a cured (set) resin paste, so that at the time of the lamination, adverse effects of the prepreg sheet (which is disposed above the printed resistor) on the printed resistor can be suppressed (Japanese Patent Unexamined Publication No. 52-140866).
When the multi-layer printed circuit board is to be produced with the inner-layer board (which has the printed resistors mounted thereon) incorporated therein, the resistivity of the printed resistor in the resultant multi-layer printed circuit board may deviate greatly from a target value, because the resin constituting the printed resistor is softened during the step of applying the heat and pressure. To avoid this, there has been proposed a printed circuit board with self-contained resistors in which a paste for a polymer thick-film resistor, which is composed mainly of a thermosetting resin polymerizable upon reaction of a methylol group, a thermosetting resin polymerizable upon ring-opening reaction of an oxirane ring, or a mixture of these two resins, is applied between terminals of conductor circuits formed on a board so as to provide a polymer thick-film resistor bridging between these terminals, and then a protective film is coated onto the surface of this resistor, and then this board is integrally combined with prepreg sheets (Japanese Patent Unexamined Publication No. 1-295482).
With the high-density packaging (mounting) and high-speed operation of electronic devices, there has also arisen a new problem that one electronic device causes jamming (radio trouble) on another electronic device, and this problem has become a familiar one. The jamming is regulated or restrained under various international rules, and in Japan, also, the regulations against radiation noises have been made under the self-imposed control advocated by Jamming Self-imposed Control Institute for Information Processing Systems or the like (VCCI), and it is expected that this control will be strengthened from now on.
Further, with the high-density and high-speed design, the spacing (distance) between parts, as well as the spacing between the conductors on the printed circuit board, has been getting smaller and smaller, and as a result a static coupling is promoted, which causes a problem that a noise trouble is liable to occur in the electronic device.
Various such problems of jamming and noise trouble have heretofore been dealt with by a suitable selection of electronic circuits and electronic parts, the use of a noise filter, the shielding, the grounding, the wiring, and so on. However, it has been difficult to meet with the above regulation.
Therefore, in order to overcome these problems, there has been developed an electromagnetic wave shield printed circuit board in which an insulating layer (interlayer insulating layer), as well as a shield layer of a copper paste (which is screen-printed) are formed on the surface of a conventional printed circuit board, and then an overcoat is formed (Hanyu et al., National
Technical Report, Vol. 35, No. 4, page 76 to 82, 1989).
Due to the following effects, this electro-magnetic wave shield printed circuit board prevents jamming and noise troubles:
(1) When shield layers are to be provided on a multi-layer printed circuit board, shield layers of a copper foil are usually formed respectively on upper and lower surfaces of an inner conductor circuit. With these shield layers, the conventional shield effect is obtained.
(2) A bypass capacitor is additionally provided between the shield layer of a copper paste, connected to a ground circuit, and signal circuits (the conductor circuits on the surface), and therefore the energy, which would reside on the signal circuits on the surface as in the conventional printed board, flows to the ground via this bypass capacitor. Thus, the effect by the capacitor is obtained.
(3) The shield layer forms the ground at a uniform distance relative to each signal circuit, and the shield layer is connected to the ground circuit in a multi-contact manner, and also covers a generally entire area of the signal circuits. Therefore, the ground circuit can be rendered into a low-inductance nature, so that high-frequency components of the energy hardly reside on the ground circuit, thereby hardly causing an unnecessary radiation of the electromagnetic wave. Thus, a ground reinforcing effect is obtained.
As described above, although the conventional printed circuit board with the self-contained printed resistors can achieve the high-density packaging to provide a thin and small-size construction, the electro-magnetic wave shield effect is insufficient. Further, the electromagnetic wave shield printed circuit board does not have a sufficient space for mounting the parts (particularly, the chips) on its surface, and therefore it has a disadvantage that the enhancement of the packaging density is limited.